Alternators convert mechanical rotational motion into electrical energy. In vehicles, such as cars and trucks, alternators are used to convert a portion of the power generated by the vehicle's internal combustion engine into electrical energy which is used to charge the vehicle's battery and power the vehicle's electrical systems. Depending on the application, the alternator has to reliably generate a significant amount of electrical power.
The alternator converts the input mechanical power applied at the rotor shaft into electrical DC output power. The conversion is less than perfect with losses occurring during the conversion process. The main losses are: joules winding losses that happen in the stator windings and the field coils, electromagnetic losses that occur in the stator stack and rotor claw pieces, electrical losses present in the rectifier assembly due to the voltage drop on the semiconductor material of the diodes, mechanical losses (bearings, alternator drive belt), ventilation (fan) losses, belt drive losses, etc. Typically, a claw type alternator has the efficiency within the 65% to 85% range. The remaining difference to 100% represent the above mentioned conversion losses that are ultimately present in the alternator as unwanted heat.
The conversion losses raise the temperature of critical components of the alternator to the point of reducing the reliability of the unit.
The common practice is to cool the brushless alternator through the use of one or two shaft mounted external fans (self ventilated/auto ventilated design).
The cooling fans are usually the radial type, also called centrifugal type and pull cooling air into the alternator. The air flows axially through the alternator and is expelled outside the unit by the fan. Along this cooling path, the air “picks-up” heat gradually becoming hotter and hotter, being its hottest when it exits the unit. A temperature map of the alternator reveals an uneven distribution of temperatures with the fan end region of the alternator running hotter relative to the opposite end by up to 35° C. Therefore special attention must be given to the drive end ball bearing(s) temperatures as they have been shown to run hotter by approximately up to 35° C. than the corresponding components from the other side (the bearing from the rectifier end). Moreover, the typical operating environment of an alternator is very warm—engine compartments with typical temperatures of in excess of 90° C. which further exacerbates the problem at the fan end (drive end) of the unit.
Standard practice is to use a light (low rotational inertia) aluminum alloy fan that can survive up to 200° C. temperatures—common at the fan end of the unit, without shape deformation or warping. Aluminum alloy is a highly conductive material and the overheated air received by the radial blades region “travels down” towards the fan center, overheating the fan hub which further transfers the unwanted heat to the front bearing(s). The diameter and width of the fan is limited by the volume available for the alternator. Therefore the limited size cooling fan needs to provide a high level of cooling power to the hot running components of the alternator on both fan side (drive end) and the opposite side (rectifier end). It is especially desirable to have an aluminum fan which more efficiently cools the drive end ball bearing(s), therefore keeping the mentioned component well under the safe operating temperatures.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.